Screen for cathode ray tubes



`Maly 30, 1939. H. w. KAUFMANN SCREEN FOR CATHODE RAY TUBES Filed Aug. 3l, 1937 INVENTOR.

l HENRY W KAUFMANN; BY

ATTORNEY.

Patented May 30, 1939 UNITED STATES PATENT OFFICE 2,160,022 scannN ron, cA'rnona an 'runas tion of Delaware Application August 31, 1937, Serial No. 161,732

9 Claims. (Cl. 25o-27.5)

My invention relates to luminescent screens for use in cathode ray apparatus. such as oscillosraph tubes and television receiving tubes.

In the conventional cathode ray tube the uorescent screen is often made of such materials as zinc ortho-silicate which when suitably activated, such as by the addition of manganese, produce a screen having high luminous eiliciency with low input power. When a cathode ray beam of relatively low current density impinges on such a screen with moderate velocity the luminous trace is clearly visible even under conditions of moderate external illumination. To obtain a luminous trace of greater light intensity the beam current, the beam velocity, or both, must be increased, but the resulting increase in luminosityis not proportional to the increase in current density and velocity of the beam, as the screen becomes saturated at a point beyond which it is diihcult to obtain greater luminosity. Moreover, the input power, which is determined by the beam current and beam velocity, must be limited, as overheating and loss of screen elciency by decomposition of such a screen occurs with relatively low input power. The conventional screen usually does not -produce a white light, a screen of zinc ortho-silicate activated by manganese having a greenish tinge which makes it undesirable i'or television use where a definite esthetic eect is desired.

'Ihe principal object of my invention is to provide a luminescent screen giving a pure white light and of greater luminosity than screens of the conventional type. A'further object is to provide a screen which will withstand intense bombardment, and is stable and of relatively long life.

In accordance with my invention I provide a screen for cathode ray tubes or similar use, comprising a mechanically strong foundation, preferably of metal, having thereon a multiplicity of minute rodlets made of refractory material of low hat conductivity, projecting side by side and supported at their proximal ends by the founda- ,A tion. The free or distal ends of the rodlets may be enlarged to form a multiplicity of minute mushroom-shaped projections from the foundation. The 'rodlets or mushroom-shaped projections are mounted with their free ends spaced so that the ends may be heated almost instantaneously to incandescence by the cathode ray beam without appreciable loss of heat 'oy conduction to adjacent areas of the screen. When the cathode ray beam is removed the incandescence decreases rapidly because of loss of heat by radiation. The incandescent ends of the rodlets give a pure white light of great intensity.

'I'he above objects and other objects, features and advantages of my invention will appear from the following description taken in connection with the accompanying drawing in which,

Figure 1 is a longitudinal view, partially in cross section, illustrating my improved luminescent screen incorporated in a cathode ray tube;

Figure 2 is a fragmentary view, in cross section and on a greatly enlarged scale, of the screen; v

Figures 3, 4, 5, and 6 are sectional views of a portion of the screen shown in Figure 2 illustrating four steps of preparation; and

Figure 7 is a view showing a preferred method cf carrying out a step in the manufacture of the luminescent screen.

In the illustrative embodiment of my invention shown in Fig. 1 the tube incorporating my new and improved luminescent screen comprises a highly evacuated glass envelope l with a tubular arm or neck section enclosing a conventional type electron gun and a spherical section enclosing` a luminescent screen assembly 2 so positioned that its front surface may be scanned by a beam of electrons from an electron gun and that the luminescent image produced by the scansion of the electron beam over the surface of the screen may be viewed or projected through a portion of the spherical section. It is therefore desirable to construct that portion of the spherical section opposite the luminescent screen of transparent optically uniform material.

The electron gun assembly is of the conventional type and comprises a cathode 3 from which an electron stream may be drawn, a control electrode 4 connected to the usual biasing battery through a source of fluctuating potential whereby the intensity of the electron stream may be modulated, and a first anode v5 maintained positive with respect to the cathode 3. 'I'he electron stream leaving the first anode 5 is accelerated and concentrated into an electron scanning beam focused on the front surface of the luminescent screen 2 by a second anode 6 which is preferably a conductive coating on the surface of the envelope I near the neck of the bulb, but removed from that portion of the spherical section through which the image produced on the luminescent screen may be viewed cr projected. 'I'he rst anode 5 and the second anode 6 are maintained at the required positive potentials with respect to the cathode 3 by the battery 1. The type of electron gun here described is highly efilcient and is capable of producing a cathode ray beam carrying as much as 10 ma. In practice the battery 8 supplies an anode potential of 6,00010,00 volts so that the beam may be considered as capable of delivering from 60-100 watts at the surface of the luminescent screen 2. Conventional deecting means, such as deflection coils 8 and 9, may be used to sweep the beam ina horizontal and vertical plane to scan the luminescent screen, but it is obvious that conventional electrostatic deflection plates may be substituted for the deection coils, if desired.

In accordance with my invention the luminescent screen which is scannedby an electron beam to produce a highly luminescent image includes, as shown in Figure 2, a foundation member I0 which is mechanically strong and which has a multiplicity of minute rodlets or projections II of some refractory material, such as a refractory oxide, with their distal ends out of contact with one another. The foundation member I0, preferably is foraminous, with a multiplicity of small apertures, and' may conveniently be a thin foraminous metal plate or a wire mesh netting having a woof and warp of wires spaced from one another to provide a multiplicity of apertures or interstices in which the proximal ends of the refractory oxide projections are supported. I have found that a netting having -200 interstices per linear inch is satisfactory for this purpose. The rodlets il are firmly fixed in the interstices of the foundation member I Il. For some applications of my device I have found it desirable to provide these rodlets with refractory caps I2, as shown in Figs. 2 and 7.

As examples of materials which may be used for the purpose of forming the oxide rodlets II and the caps I2 are salts, such as the nitrates of thorium, cerium, calcium, and manganese, which upon heating are converted into the oxide with increase in volume. When such salts are decomposed by heat they will produce a porous mass which is stable when raised to incandescence by bombardment of the cathode ray beam. The preferred material, however, is thorium nitrate, which may be easily decomposed by heat into thorium oxide. I have used with satisfactory results a solution of 100 grams of thorium nitrate in 100 cubic centimeters of water, to which is added some material which will serve as a flux, such as approximately 1/2 gram of boric oxide. The use of such a ux produces a finished screen which is more rugged under mechanical shock. The solution is then applied to the foundation member I0 by painting, dipping, spraying or any convenient method, whereby the apertures or interstices of the foundation are filled or substantially'iilled with the solution. The interstices are so small that they hold the solution by capillary attraction, little or none of the material remaining on the metal foundation between the interstices. The foundation member is painted, sprayed or dipped repeatedly, and dried in air until the interstices of the foundation member I0 are completely filled. The foundation member I0, with individually supported and separate plugs of nitrate I3 in the interstices, is then rapidly heated, preferably by heat generated by an electric current passing directly through the wires constituting the mesh member, as the best results are obtained when the nitrate plugs are heated from the metal of the foundation. This may be accomplished conveniently, as shown in Fig. 7, by clamping heavy copper electrodes I4 to the oposite sides of the foundation member and passing an electric current through the mesh between the oppositely disposed electrodes. It is desirable that the foundation member I0 be brought to a high and uniform temperature, such as about 1000 C., in as short a vtime as possible, which ycauses the individual nitrate plugs I3 to decompose into the corresponding oxides. For the purposes of obtaining uniform heat distribution throughout the mesh and especially near the edges of the mesh not held by the electrodes, I prefer to use metal guard strips I5 co-planar with and close to each edge of the member I0. These strips may be formed from the same material as the foundation member I0 and are clamped between the electrodes I5 and heated to the same temperatureas the foundation member I0 by the current supplied to the electrodes.

When the wires constituting the foundation member I0 are heated, such as by passing an electric current through these Wires, the nitrate plugs are converted into oxides, as indicated in Figs. 4 and 5. The portion of the nitrate plugs contacting the walls of the interstices is immediately melted and decomposed into the oxide. Heat is conducted through the oxide thus formed to the inner portion of the plugs and causes the nitrate constituting the inner portion of the plugs to be melted and decomposed. The nltrate plugs are in this way progressively decomposed into thorium oxide from those portions of the plugs which are in contact with the foundation inwardly of the plugs. Simultaneously with this action the plugs expand as the nitrate is converted into the oxide and form the rodlike members which contain in their tips a small portion of melted nitrate I6 which, as the process continues, is converted into the oxide. By such rapid heating it is possible to obtain, in a few seconds, from the plugs of nitrate I3 rodlets II of porous oxide (Fig. 5) which are supported at their proximal ends by the foundation member Luminescent screens made in this manner have the desired characteristics, but for certain purposes I prefer to form on the distal ends of the rodlets II cap portions I2 which tend to overlap those portions of the foundation member III between its interstices. The caps I2 may be provided by spraying the nitrate solution mentioned above over the exposed ends of the rodlets II formed as above described. In spraying the nitrate solution the foundation member carrying the individual oxide rodlets'is preferably inclined at an angle to the direction of the spray to minimize the deposition of the material on the foundation member between the oxide rodlets. The caps I2 are then formed by decomposing the nitrate on the ends of the rodlets into the oxide by heating the assembly for a few minutes in a furnace operated at a temperature of approximately 1000 C. This heating causes the nitrates to be converted into oxides, thereby converting the rodlets II to mushroom-shaped projections I1 with the individually supported and mutually separated caps I2. Any nitrate which has been inadvertently deposited on the foundation member I0 between the individual plugs of oxide by the spraying process is also converted into the oxide, but this is immaterial, inasmuch as the caps I2 are suiliciently close togetheras to shield this portion of the screen from the cathode ray beam during' operation.

A screen made in accordance with my invention may be conveniently mounted in a frame I8 so that the distal ends of the rodlets or projections face the electron gun. The frame I9 extends around the edges of the screen 2. supporting it in such a manner that it will be substantially fiat over its entire surface. For purposes of rigidity I prefer to mount withinl this frame and behind the screen 2 a relatively thick sheet I9 of mica or other heat insulating material corresponding in area and shape to the screen 2. The mica sheet I9 may be spaced from the screen 2 by strips lof mica 20 to decrease thermal conductivity from the screen 2. 'Ihis assembly is mounted in the cathode ray tube. as shown in Fig. 1, by support wires 2|, at least one of which serves as an electrical conductor and is connected to the foundation member I9 and to the second anode 9.

The beam is deflected to fall upon the distal .ends of the rodlets in succession and to trace a pattern on the screen by the deilecting coils 8 and 9. Since the screen 2, as shown in Fig. l, has its effective surface in a plane which is at an acute angle to the electron gun axis for the purpose of viewing the scanned side of the screen,

the pattern swept out by the beam will be in the form of a quadrilateral having two opposite sides parallel and the other sides inclined toward each other at a slight angle. This eilect is known as keystoning and may be obviated in ways well known in the art as, for example, by properly shaping the magnetic pole pieces about which the coils 8 and 9 are wound. Since the individual rodlets or projections of the screen 2 are spaced from those which are adjacent, the heat generated in these rodlets or projections by the impact of the beam is lost almost entirely by radiation, the conduction from one to another being necessarily through the rodlets or projections and through the foundation member I0. This conduction is very small due to both the low thermal conductivity of the porous oxide and to its small cross sectional area transverse to the direction of the heat ow. Since radiation is practically the only channel of heat loss and since a large amount f power is expended in the small mass comprising the ends of the individual elements, the distal ends of the rodlets or projections are excited to incandescence almost instantly. Once excited they continue to be incandescent until the loss of heat by radiation, which continues over a materially greater period than the excitation period, reduces them to darkness. In the use of luminescent screens made in accordance with the present invention, the beam is constantly moving from f one elemental area to another and the mass of material in the area scanned by the beam is so small at any one time that the mass becomes incandescent almost instantly. Furthermore, conduction to adjacent elements is so restricted that heat loss by radiation coolsthe bombarded area before the heat can spread to adjacent areas thereby limiting the spreading of the luminescent area to a minimum. With screens of the type illustrated, itis possible to obtain an emciency of approximately 39 lumens/watt at a brightness of approximately 300 candle power/square centimeter. Thus with an input power of around 60 watts, the image produced on a screen of this type is so intense that it cannot be viewed directly. It may, however, be projected by a suitable lens system upon a projection surface having a high coefficient of reflection. It is, therefore, possible when using screens of this type to project white light images on a distant projection surface even under conditions of relatively high external illumination. Such a luminescent screen is therefore entirely suitable for projection of television images.

Fromv the foregoing description it will be apparent that various other modifications may be made in my invention without departing from the spirit and scope thereof and I desire, therefore, that only such limitations shall be placed thereon as are necessitated by the prior art and set forth in the appended claims.

I claim:

1. A luminescent screen comprising a metal foundation member having a multiplicity of perforations, and rodlets of a refractory oxide xed in said perforations to project extending substantially from a surface of said foundation'member with their distal ends spaced from one another and leaving voids therebetween.

2. A luminescent screen comprising a foraminous foundation member, and projections of a refractory oxide secured in a foramina of said member to extend beyond the surface of said foundation with their distal ends spaced from one another and leaving voids between the said distal ends.

3. A luminescent screen comprising a perforated metal mesh foundation, and a multiplicity of projections of a refractory oxide with their proximal ends enclosed in and supported by the mesh and their distal ends spaced from one another and extending from the said foundation for a substantial distance.

4. A luminescent screen comprising an intersticed metal foundation, anda multiplicity of refractory oxide projections supported by their `proximal ends in the interstices of said foundation and their distal ends extending substantially beyond the surface of said foundation.

5. A luminescent screen comprising an intersticed metal foundation, a multiplicity of rodlets of refractory oxide supported at one end in the interstices of said foundation and extending substantially from said foundation, and a cap of refractory oxide on the distal ends of each of said rodlets. 1

6. The method of forming a luminescent screen including a perforated foundation comprising substantially filling the perforations of said foundation with a readily oxidizable material selected from the group consisting of salts of thorium,

cerium, calcium, and manganese decomposable into oxides by heat, and forming a multiplicity of rodlets extending from said foundation by rapidly heating said foundation.

7. 'I'he method of forming a luminescent screen including a perforated metal foundation comprising substantially filling the perforations of said foundation with thorium nitrate to form plugs of the nitrate in the perforations and progressively decomposing the plugs inwardly from the surface of the plugs in contact with the foundation by passing electric current through said foundation to heat said foundation rapidly.

8. The method of forming a luminescent screen including a perforated metal foundation comprising substantially filling the perforations with a solution of thorium nitrate, heating the foundation at a low temperature to form solid plugs of thorium nitrate in said perforations and progressively decomposing the plugs of thorium nitrate to porous thorium oxide inwardly from the surface of the plugs in contact with the foundation by subjecting said plugs to a higher temperature.

said plugs to be converted into porous thorium oxide rodlets, applying thorium nitrate to the distal ends of said rodlets and heating said ends to convert said last applied thorium nitrate into porous thorium oxide.

HENRY W. KAUFMANN. 

